1. Field of the Invention
The present invention relates to a flatness measuring apparatus.
2. Description of the Related Art
As LSI patterns have been becoming increasingly fine in recent years, a reduction in the focal depth of optical exposure apparatuses (optical aligners) has been observed. Namely, while the wavelength must be reduced to transcribe (expose) a fine pattern, the shorter wavelength results in a reduction in the focal depth in the optical exposure apparatus since the focal depth is in proportion to the wavelength. For this reason, the quality of transcribing is determined by whether or not indentations and projections at the surface of the semiconductor wafer are contained within the focal depth, and the requirements imposed with respect to the flatness of the semiconductor wafer upon which the pattern is transcribed (exposed) are becoming increasingly rigorous. For instance, if the surface of a semiconductor wafer has undulations and the magnitude of the undulation exceeds the focal depth there will be a loss of clarity in transcribing.
In the prior art, in order to measure the flatness of a semiconductor wafer, a sensor is positioned facing opposite the two surfaces of the semiconductor wafer by holding a portion of the semiconductor wafer and the distances to the individual measuring points of the wafer are measured by scanning with the sensor. For instance, the data collection by the sensor is made to correspond to a 1 mm pitch in the transcribing area, the height distribution at the semiconductor wafer surface is measured over this 1 mm pitch and, based upon the results of the measurement, the flatness of the semiconductor water is measured.
FIG. 4 illustrates transcribing areas at a semiconductor wafer. The surface of the semiconductor wafer 20 is divided into a plurality of transcribing areas (20-1.about.20-N) (N is a positive integer) as illustrated in FIG. 4, for instance, and the flatness of the semiconductor wafer is measured in the individual transcribing areas.
FIG. 5 illustrates measuring points in a flatness measuring method in the prior art, in FIG. 5, which is an enlargement of one of the transcribing areas (e.g., 20-n) (n is a positive integer) in FIG. 4, a plurality of measuring points are set over for instance, 1 mm pitch in the transcribing area. The surface height of the semiconductor wafer 20 is measured at all the measuring points (20-n-1.about.20-n-M) (M is a positive integer).
FIG. 6 illustrates a reference flat plane used in the flatness measuring method in the prior art. Using the measurement data indicating the heights at the individual measuring points, a reference flat plane R1 that represents the transcribing area (20-n) is calculated through a method of least squares or the like. The measurement of flatness is implemented using quantities of displacement of the individual measuring points (20-n-1.about.20-1-M) relative to the reference flat plane R1.
An optical exposure apparatus transcribes by detecting an optimal focal plane in each of the transcribing areas (20-1.about.20-N) with a focus sensor (e.g., a stepper AF sensor) or the like to transcribe with the focus adjusted to the focal plane. FIG. 7 illustrates measuring points used during the transcribing operation performed by the optical exposure apparatus. During the transcribing operation, the heights at a plurality (4 in FIG. 7) of specific measuring points (20-n-f1.about.20-n-f4) within a transcribing area are measured by the focus sensor while the semiconductor wafer 20 is held in a wafer holder.
FIG. 8 illustrates the relationship between the reference flat plane R1 in the flatness measuring method of the prior art and a transcribing reference flat plane R2 used by the optical exposure apparatus for the transcribing operation. Using the measurement data indicating the heights at the individual measuring points, the transcribing reference flat plane R2 that represents this particular transcribing area is calculated through the method of least squares or the like. Then the transcribing is performed with the focus adjusted to the transcribing reference flat plane R2.
However, since the number of measuring points at which the height is measured during the flatness measurement is not the same as the number of measuring points at which the height is measured during the transcribing operation, there is a problem in that the reference flat piano used for the measurement of the flatness of the semiconductor wafer 20 does not always match the reference flat plane (focal plane) used during the transcribing operation. As a result, the focusing state on the semiconductor wafer 20 during the transcribing operation cannot be assured with a high degree of accuracy.